SYDNEY - Oct 21 (Reuters) - Thousands of people rallied around Australia on Saturday urging the legalization of same-sex marriage, one week before final ballots can be submitted in a contentious postal survey on the issue that has divided the country.

The largest crowd was in Sydney, where organizers said between 5,000 and 10,000 people gathered in front of Central Station before marching along one of the city's biggest roads to Victoria Park.

"It's a good reflection of the enthusiasm of people," Australian Marriage Equality's Tiernan Brady said. "They are very determined, very positive and not complacent."

Other rallies in favour of same-sex marriage were held in the northern city of Brisbane and the central hub of Alice Springs.

Rallies organised by the Coalition for Marriage, the lead campaigner against same-sex marriage, also were held across the country.

The coalition, which includes the Australian Christian Lobby and other religious groups, encouraged those who haven't returned their surveys to do so.

"We're so pleased so many people have engaged with this process and we encourage those who haven't to tick 'no' and put it in the post," spokeswoman Monica Doumit said.

Though the postal ballot is non-binding, a "yes" vote is expected to lead to the legalization of same-sex marriage which could further fracture the government of Prime Minister Malcolm Turnbull.

Ballots were mailed out from Sept. 12, with the Australian Bureau of Statistics recommending all votes be returned via the postal service by Oct. 27.

The latest update from the ABS, issued on Oct. 17, showed almost 11 million postal votes had been returned, about 68 per cent of the total distributed.